The present invention relates to a fibre feeding device for feeding optical fibres into a tubular element or into some corresponding element, either singly, loosely or closely connected together, this latter being preferred.
It is known to install optical fibres by means of a blowing technique. In order to allow an optical fibre or several optical fibres contained in an optical fibre ribbon to be blown into a tubular element, duct, or the like, it is essential that the friction between the optical fibre or fibre ribbon and the inner surface of the tubular element is very low, that the presence of static electricity is negligible, and that the air resistance is sufficiently large to enable the optical fibre(s) or fibre ribbon to be advanced even at moderate air propulsion speeds. Hitherto, the solution to this problem has been achieved by applying to the optical fibre(s) 1 or to the optical fibre ribbon a diameter-increasing layer 2 that includes friction reducing material. In the case of mutually combined or closely connected optical fibres 3, 4, several of these fibre combinations have been orientated in a rotational-symmetrical bundle so as to have the same flexural resistance in all planes, and have been provided with a diameter-increasing layer 5, 6 of glass beads, for instance. In order to be able to use optical fibre ribbon techniques 7-9 and utilise the advantages that this would afford in respect of welding and the establishment of electrical contacts without requiring additional material for rounding the cross-section of the ribbon, and for reducing friction and increasing air resistance, a thin Kevlar(copyright) wire or filament 10 may be wound around the optical fibre ribbon. This wire can be readily removed at the ends of the optical fibres at a later time, when the ends are to be welded or when electrical connections shall be established. See FIGS. 1A-G.
When an optical fibre 11 or an optical fibre ribbon shall be fed into a tubular element, e.g. ducting, with the aid of compressed air, the optical fibre or ribbon is fed by an optical fibre feeding device into a pressurised space 12 that is connected to the tubular element 13 into which the optical fibre shall be blown. In so doing, it is necessary to apply some form of force that will overcome the backward ejection force acting on the optical fibre in a direction opposite to the feed direction. This has been achieved in the case of several systems, by allowing the optical fibre 11 to pass between two feed rollers 14, 15 that have a soft surface coating. These rollers or wheels may be electrically operated, although they may also conceivably be operated by the pressured air used to propel the optical fibre, so as not to damage the fibre should it be prevented from moving along the duct for some reason or other. A pressure sensor 16 may be provided in the space 12 so as to detect any pressure changes that would result from forward movement of the fibre through the duct being prevented. An optical fibre feeding device designed to operate in accordance with this fibre feeding technique would be relatively complicated and would also require the supply of electrical energy in addition to the compressed air supply for propelling the optical fibre. See FIG. 2.
The aforesaid backward ejection force in the duct can be partially overcome with the aid of an ejector effect generated by the compressed air 17 supplied, by allowing a leakage flow 18 to pass through an optical fibre blow pipe 19 that is designed to function as a Venturi tube, where the air velocity is high and the pressure is low. The blow pipe may be provided with adjustable outlet valves 20 on the downstream side, for controlling the leakage flow. In this way, the optical fibre or fibre ribbon is subjected to an additional propulsion force when introduced into the blow pipe. The leakage flow results in a lower pressure in the blow pipe, a factor which may allow the blow length to be reduced. See FIG. 3.
With the intention of simplifying handling of an optical fibre feeding device that includes a blowing passage for feeding one or more optical fibres into a tubular element, duct or the like, the blow passage can be opened to allow different types of optical fibres to be fed thereinto. The optical fibre feeding device comprises a body that includes a groove or channel into which there can be inserted a bar that includes one or more longitudinally extending blow passages, where the cross-section of each passage is adapted for a given type of optical fibre. Holes for delivering and removing compressed air used to propel the optical fibre(s) are provided in the bottom surface of the groove or channel in the close proximity of its respective rear and front edges. With an optical fibre placed in the blow passage and with the bar placed in the groove/channel in the body of said device, the optical fibre will be propelled forwards when compressed air is delivered through the holes at the rear edge of the groove/channel, by virtue of the blow passage functioning in the manner of a Venturi tube, wherewith a small leakage flow in the blow passage will cause transportation of the optical fibre through the fibre feeding device and into a blow pipe through the medium of an ejector effect. Because the bar is removable and is provided with grooves or channels of mutually different sizes, the fibre feeding device can be used for different types of optical fibres and also enables optical fibres that have already been connected electrically to be fed into the tubular element or duct. According to the invention, the blow passages may be given a rectangular cross-section, so as to enable a non-rounded optical fibre ribbon to be blown into the tubular element or duct by means of the fibre feeding device.
The invention will now be described in more detail with reference to a preferred embodiment thereof and also with reference to the accompanying drawings.